Connectivity Fault Management (CFM), as described in IEEE 802.1ag, is a key component of operation, administration, and maintenance for carrier Ethernet. IEEE 802.1ag specifies protocols, procedures, and managed objects for end-to-end fault detection, verification, and isolation. IEEE 802.1ag establishes managed objects, called Maintenance Associations (MAs), to verify the integrity of a single service instance by exchanging CFM messages. The scope of an MA is determined by its Management Domain (MD), which describes a network region where connectivity and performance is managed. Each MA associates two or more Maintenance Association Endpoints (MEPs) and enables Maintenance Association Intermediate Points (MIPs) to support fault detection and isolation.
A continuity check protocol is used for fault detection. Each MEP periodically transmits Connectivity Check Messages (CCMs) and tracks CCMs received from other MEPs in the same maintenance association.
FIG. 1 illustrates the format of an existing CFM Protocol Data Unit (PDU) 10. A common CFM Header consists of a Management Domain (MD) level field 11, Version field 12, OpCode field 13, Flags field 14, and First Time, Length, and Value (TLV) Offset field 15. The Flags field 14 of the Common CFM Header is currently split into three parts:
1. RDI field (one bit, the most significant bit);
2. Reserved field (4 bits); and
3. CCM Interval field (the least significant three bits).
Provider Backbone Bridging-Traffic Engineering (PBB-TE), as described in IEEE 802.1Qay, was designed to provide full traffic engineering of paths in a bridged network. PBB-TE eliminates the need for backbone devices to perform learning and flooding. Instead of using Multiple Spanning Tree Protocol/Rapid Spanning Tree Protocol (MSTP/RSTP) for loop avoidance, PBB-TE uses a management plane or an external control plane to create static filtering table entries in the component bridges.
PBB-TE is a connection-oriented Ethernet technology that uses a statically configured tuple consisting of the Backbone Destination Address (B-DA), Backbone Source Address (B-SA), and Backbone VLAN ID (B-VID) to create a PBB-TE path. The provisioned path is called an Ethernet Switched Path (ESP). Two co-routed point-to-point ESPs with the same Customer Backbone Port (CBP) MAC addresses form a bidirectional MAC service, which is called a point-to-point Traffic Engineering Service Instance (TESI).
PBB-TE supports 1:1 bidirectional path-protection switching. Two point-to-point TESIs are provisioned. One TESI is configured as a “working” TESI and the other as a “protection” TESI. In normal conditions, traffic is transmitted over the working TESI. In the event of either a failure of the working TESI or a specific administrative request, traffic is switched to the protection TESI. Optionally, PBB-TE 1:1 protected paths may be configured to allow for load sharing. In this case, customer services denoted by an I-TAGed frame flow may be present on both TESIs in a protection group.
FIG. 2 illustrates a conventional PBB-TE protection group 20. The protection group includes a working TESI 21, a protection TESI 22, a near end (East B-Component) 23, and a far end (West B-Component) 24. The near end (East B-Component) includes Provider Network Ports (PNPs) 25a and 25b and Customer Backbone Port (CBP) 26. The far end (West B-Component) includes PNPs 27a and 27b and CBP 28. Each TESI is monitored by an independent MA, and each MA has two MEPs. One is located in the CBP 26 of the near end; the other is located in the CBP 28 of the far end. When the near end MEP detects the loss of CCMs, it notifies the far end MEP by sending a CCM with a Remote Defect Indicator (RDI) flag. Both ends are aware of the failure (either by loss of CCMs or receiving the CCM with the RDI flag), so protection switching to the protection TESI is executed on both ends. When the failure is cleared, traffic may be switched back to the working TESI 21 or may stay in the protection TESI 22 according to the configured mode (revertive or non-revertive).
ITU-T G.8031 defines the Automatic Protection Switching (APS) protocol and linear protection switching mechanisms for point-to-point VLAN-based Ethernet Subnetwork connections in Ethernet transport networks. Linear 1+1 and 1:1 protection switching architectures with unidirectional and bidirectional switching are supported.
The current PBB-TE draft (2.0) supports 1:1 bidirectional path protection switching based upon the ITU-T G.8031 model. The differences between PBB-TE protection functionality and ITU-T G.8031 protection functionality are:                ITU-T G.8031 defines the APS protocol as the signaling Protocol Data Unit (PDU) while PBB-TE re-uses/extends the CCM PDU to avoid the unnecessary complexity of an additional signaling PDU.        In PBB-TE, an “out-of-band” management system is assumed to coordinate the two ends of the protected service belonging to a single domain.        In PBB-TE, a protected flow is identified by a TESI while in G.8031 the protected flow is identified by a VLAN ID (VID).        
In 1:1 bidirectional protection switching, a mismatch between the bridge/selector positions of the near end 23 and the far end 24 can occur. To maintain the proper operation of the network, this mismatch should be detected and reported to the network operator. Then the network operator can clear the defect. There are two types of mismatch in 1:1 bidirectional protection switching:                Protection switching incomplete mismatch; and        Working/protection configuration mismatch.        
Still referring to FIG. 2, a scenario is depicted in which a protection switching incomplete mismatch occurs. In this example, due to a hardware malfunction, the near end (East B-Component) 23 fails to switch over, but it sends an RDI to the far end (West B-Component) 24. The far end switches to the protection TESI 22 while the near end is still in the working TESI 21. Similarly, a mismatch can also occur when the near end switches to the protection TESI, but the far end fails to switch when it receives the RDI.
A mismatch can also occur because of a wrong configuration. For example, one end may be configured to send traffic on the working TESI 21 while the other end is configured to send traffic on the protection TESI 22. Similarly, one end may be configured in the revertive mode while the other end is configured in the non-revertive mode. In this case, the mismatch occurs when a failure is cleared.
Based on the existing mechanisms, there are two ways to address the mismatch problem, but both are undesirable in a PBB-TE environment.
First, the APS protocol can be utilized to detect the mismatch (as in G.8031), but this approach is too complicated. In G.8031, the APS protocol is designed for Linear 1+1 and 1:1 protection switching architectures with unidirectional and bidirectional switching. Since PBB-TE only focuses on 1:1 bidirectional protection switching, and PBB-TE already assumes an “out-of-band” management system to coordinate both ends in a single domain, the APS protocol brings a great many duplicated and unnecessary functionalities. In addition, adding the APS protocol to a bridge creates major architecture changes.
FIG. 3 illustrates an example of how a mismatch in 1:1 bidirectional protection switching may be detected by an “out-of-band” Operations Support System/Network Management System (OSS/NMS) 31. Although the mismatch can be detected by using the APS protocol, without APS, the OSS/NMS can be utilized. In step 32, the OSS/NMS requests the selector/bridge position from both the near end (East B-Component) 23 and the far end (West B-Component) 24. At step 33, the near end and far end report their selector/bridge positions to the OSS/NMS. At step 34, the OSS/NMS compares the reported positions to detect a mismatch. For the PBB-TE environment, this process is too slow, and it has the additional disadvantage of having to be initiated proactively by an operator.